Method of and circuit for brown-out detection

ABSTRACT

A circuit and method for detecting a brown-out condition and providing a feed-forward transfer function in a power supply circuit. A comparison circuit is coupled to a delay element through a latch. A second delay element is connected between the first delay element and an input of the latch. The output of the first delay element is connected to a clamping circuit via a logic circuit. A first voltage is compared with a reference voltage to generate a comparison voltage, which is transmitted through the latch and the first delay element. The comparison voltage is monitored at an output of the first delay element. A brown-out condition occurs if the comparison voltage being monitored at the output of the first delay element results from the first voltage being less than the reference voltage.

TECHNICAL FIELD

The present invention relates, in general, to power supplies and, more particularly, to brown-out detection and feed-forward circuitry in power supplies.

BACKGROUND

Voltage regulators that provide AC/DC rectification typically include a full wave voltage rectifier stage such as, for example, a diode bridge, a main Switch Mode Power Supply (SMPS) stage, and a Power Factor Correction (PFC) stage inserted between the line and the main SMPS. The SMPS provides regulation of an output waveform and the PFC stage draws a sinusoidal current from the line and provides Direct Current (DC) voltage to the main SMPS. For many systems to operate properly, it is desirable for the PFC stage to include a feed-forward circuit and a brown-out detection circuit. The feed-forward circuit provides an input-to-output transfer function that is independent of the input voltage which eases loop compensation whereas the brown-out detection circuit disables the converter if the line magnitude is too low. What is more, it is desirable for the brown-out detection circuit to include a hold-time component to ignore short interruptions to the mains, thereby allowing the power supply to continue operating when these short interruptions occur. Typically, brown-out detection circuits capable of ignoring short mains interruptions are comprised of filter circuits. A drawback with using filter circuits is that they degrade the response to line variations because the brown-out detection circuit and the feed-forward circuit receive input signals from a common input pin. Thus, designers have had to trade-off the performance of feed-forward circuitry with the performance of brown-out detection circuits.

Accordingly, it would be advantageous to have a circuit and method for detecting brown-out conditions that meet desired hold-time specifications and that are capable of providing a feed-forward transfer function that is independent of the line magnitude. It would be of further advantage for the circuit and method to be cost efficient to implement.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing figures, in which like reference characters designate like elements and in which:

FIG. 1 is a schematic diagram of a circuit for use in power supply applications in accordance with an embodiment of the present invention;

FIG. 2 is a flow diagram illustrating a method for detecting a brown-out condition in accordance with embodiments of the present invention;

FIG. 3 is a schematic diagram of a circuit for use in power supply applications in accordance with an embodiment of the present invention; and

FIG. 4 is a flow diagram illustrating a method for detecting a brown-out condition in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a circuit 10 suitable for use in power supply applications in accordance with embodiments of the present invention. Circuit 10 includes a brown-out detection circuit 12 and a feed-forward circuit 14 coupled to an input/output node 16. Input/output node 16 is also referred to as brown-out node and may be, for example, an input/output pin. Brown-out node 16 is coupled to an AC line through an input stage 11. Brown-out detection circuit 12 includes a circuit element such as, for example, an operational amplifier 20, a latch 22, delay elements 24 and 26, a logic circuit 28, a hysteresis circuit 30, a clamping circuit 32, and an auxiliary clamping circuit 34. Latch 22 is also referred to as a latching circuit and delay elements 24 and 26 are also referred to as delay circuits. More particularly, operational amplifier 20 has a non-inverting input coupled for receiving a reference potential V_(REF), an inverting input connected to brown-out node 16, and an output connected to an input of latch 22, to an input of logic circuit 28, and to an input of clamping circuit 32. The connection of the output of operational amplifier 20, the input of latch 22, and the input of logic circuit 28 form a node 23. An output of latch 22 is connected to an input of delay element 24 and to the reset inputs of delay elements 24 and 26 through an inverter 36. The connection of the output of latch 22 and the inputs of delay element 24 and inverter 36 form a node 25. An output of delay element 24, an input of delay element 26, and an input of logic circuit 28 are commonly connected together forming a node 29. The output of delay element 26 is connected in a feedback configuration to a reset input of latch 22. By way of example, delay elements 24 and 26 introduce a blanking delay and a monitoring window, respectively, of about 100 milliseconds (msec) each. The lengths of the blanking delay and monitoring window are not limitations of the present invention. In addition, the blanking delay and monitoring window of the respective delay elements 24 and 26 are not limited to being the same length of time. In other words, the blanking delay of delay element 24 may be longer than the monitoring window of delay element 26 or the monitoring window of delay element 26 may be longer than the blanking delay of delay element 24. Delay elements 24 and 26 may be replaced by a single delay element.

In accordance with an embodiment, logic circuit 28 comprises a two-input OR gate 38 connected to a two-input AND gate 40 where an input of AND gate 40 serves as the input of logic circuit 28 that is connected to the output of operational amplifier 20 and an input of OR gate 38 serves as the input of logic circuit 28 that is connected to the output of delay element 24 and to the input of delay element 26 to form node 29. The output of OR gate 38 is connected to the other input of AND gate 40 and the output of AND gate 40 is connected to the other input of two-input OR gate 38.

An output drive stage 41 is connected to the output of logic circuit 28. By way of example, output drive stage 41 has an input connected to the output of logic circuit 28 and an output that serves as an output node 68. In accordance with an embodiment, output drive stage 41 is comprised of series connected inverters 43 and 45. Output drive stage 41 provides a delay in an output signal traveling from the output of logic circuit 28 to output node 68. The number of series connected inverters is not a limitation of the present invention.

Clamping circuit 32 is comprised of an inverter 42 having an input connected to the output of logic circuit 28 and coupled for controlling a switch 44. The output of inverter 42 is coupled for controlling a switch 46. Switch 44 has a terminal coupled for receiving a source of operating potential such as, for example, ground, and a terminal commonly connected to a terminal of switch 46 and to an anode of a diode 48. The cathode of diode 48 is connected to the inverting input of operational amplifier 20. The output of operational amplifier 20 is coupled to a terminal of switch 46 through a diode 49. The anode of diode 49 is commonly connected to the output of operational amplifier 20 and to an input of latch 22 and to an input of logic circuit 28.

Hysteresis circuit 30 is comprised of a switch 50 having a terminal coupled for receiving a source of operating potential such as, for example, ground, and a terminal coupled to the inverting input of operational amplifier 20 through a current source 52. A control terminal of switch 50 is connected to the output of logic circuit 28.

Auxiliary clamping circuit 34 is comprised of a PNP bipolar transistor 54 having an emitter commonly connected to the inverting input of operational amplifier 20, to the cathode of diode 48, and to brown-out node 16. The collector of PNP bipolar transistor 54 is coupled for receiving a source of operating potential such as, for example, ground and the base of PNP bipolar transistor 54 is coupled for receiving a source of operating potential such as, for example, V_(DD), through a resistor 56.

Feed-forward circuit 14 includes a voltage-to-current converter 58 having an input commonly connected to the inverting input of operational amplifier 20, to the emitter of PNP bipolar transistor 54, to the cathode of diode 48 and to brown-out node 16. An output of voltage-to-current converter 58 is connected to a current mirror 60 that sinks a current I_(BO). Current mirror 60 is comprised of NPN bipolar transistors 64 and 66, where the emitters of NPN bipolar transistors 64 and 66 are commonly connected together for receiving a source of operating potential such as, for example, ground, and their bases are commonly connected to each other and to the collector of NPN bipolar transistor 64. Voltage-to-current converter 58 generates a current I_(BO) that is proportional to the voltage at brown-out node 16. The voltage appearing at brown-out node 16 is referred to as the brown-out voltage and is an averaged portion of the input voltage. Feed-forward circuit 14 makes the transfer function and the power delivery independent of the AC line level.

FIG. 2 is a flow diagram 70 illustrating the operation of brown-out detection circuit 12 in accordance with embodiments of the present invention. A voltage V_(BO) that is a portion of an input voltage from an input stage 11 is applied to brown-out node 16 (indicated by box 72). Voltage V_(BO) is compared with a reference voltage V_(REF) by operational amplifier 20, which generates a comparison voltage V_(COMP) at its output, i.e., node 23, (indicated by box 74). By way of example, reference voltage V_(REF) is one volt. If voltage V_(BO) is greater than reference voltage V_(REF,) comparison voltage V_(COMP) is a logic low voltage and if voltage V_(BO) is less than reference voltage V_(REF), comparison voltage V_(COMP) is a logic high voltage. When comparison voltage V_(COMP) is a logic high voltage, it is substantially equal to reference voltage V_(REF) plus the forward voltage drops across two series connected diodes. It should be noted that comparison voltage V_(COMP) is also referred to as an operational amplifier output signal when an operational amplifier makes the comparison.

In response to comparison voltage V_(COMP) being a logic low voltage, brown-out detection circuit 12 continues to compare brown-out voltage V_(BO) with reference voltage V_(REF) and generates or updates comparison voltage V_(BO) (indicated by decision diamond 75 and box 74). If comparison voltage V_(COMP) is a logic high voltage, latch 22, i.e., voltage V_(LATCH) at node 25, is set to a logic high voltage (indicated by decision diamond 75 and box 76). In addition, each time the output (node 25) of latch 22 transitions to a logic high voltage level, a blanking period starts (indicated by box 77). As discussed above, the length of the blanking delay may be, for example, 100 msec. However, the duration or length of the blanking delay is not a limitation of the present invention. The voltage at node 29 is a logic low voltage during the blanking time and turns to a logic high voltage at the end of the blanking time. The blanking time is a predetermined delay that precludes any transient changes in voltage V_(BO) from being detected and identified as a fault or brown-out condition. At the end of the blanking time, brown-out detection circuit 12 starts a monitoring window during which comparison voltage V_(COMP) is monitored (indicated by box 78). If comparison voltage V_(COMP) is low for the whole monitoring window (indicated by decision diamond 79), brown-out detection circuit 12 resets latch 22 and delay elements 24 and 26 (indicated by box 80) after the monitoring window ends. Then brown-out detection circuit 12 continues to compare brown-out voltage V_(BO) to reference voltage V_(REF) and generates or updates comparison voltage V_(COMP) (indicated by decision diamond 75 and box 74).

If comparison voltage V_(COMP) is high at any time during the monitoring window, switches 44 and 50 are closed, switch 46 is opened, latch 22 is reset, and delay elements 24 and 26 are reset (indicated by box 81). Thus, latch 22 is reset at the end of the monitoring window delay or if a brown-out fault or condition has occurred. The voltage level at node 29 indicates whether a monitoring window is operating within brown-out detection circuit 12. The voltage at node 29 is at a logic high voltage level during the monitoring window and a logic low voltage level during the rest of the time, including during the blanking delay.

It should be noted that a logic low voltage level at the control terminals of switches 44, 46, and 50 opens them and a logic high voltage level at the control terminals of switches 44, 46, and 50 closes them.

If voltage V_(BO) is higher than reference voltage V_(REF), brown-out detection circuit 12 closes switch 46 and opens switches 44 and 50. If voltage V_(BO) becomes less than reference voltage V_(REF), switch 46 being closed and switch 44 being open, the inverting input of operational amplifier 20 is clamped at a voltage that is about two diode voltage drops below comparison voltage V. Closing switch 46 and opening switch 44 clamps the voltage at the inverting input of operational amplifier 20 at a voltage V_(CLAMP) that is about the same voltage as reference voltage V_(REF). Clamping the voltage at the inverting input of operational amplifier 20 forces a minimum voltage on brown-out node 16 and therefore on the inverting input of operational amplifier 20. Closing switch 50 enables current source 52 to conduct a current I_(HYS), which provides programmable hysteresis. By way of example, current source I_(HYS) conducts a current of about 7 microamperes. When the voltage appearing at node 29, i.e., the output of delay element 24, is at a logic high voltage level and comparison voltage V_(COMP) appearing at the output of operational amplifier 20 is at a logic high voltage level, the voltage at the output of logic circuit 28 and output voltage V_(OUT) at output node 68 are set to logic high voltage levels. Thus, the voltage at node 29 and the comparison voltage V_(COMP) are monitored and a brown-out condition occurs if the voltage being monitored at node 29 and the comparison voltage results from voltage V_(BO) being less than the reference voltage V_(REF).

When a brown-out condition occurs, circuit 10 clamps brown-out node 16 at a clamping voltage so that it does not fall below reference voltage V_(REF). This voltage is, however, low enough so that voltage V_(COMP) is a logic high voltage.

It should be noted that auxiliary clamping circuit 34 is included to provide an auxiliary clamping voltage that clamps, maintains, or holds brown-out node 16 at a voltage level that is less than or equal to reference voltage V_(REF) when the voltage level of the supply voltage for circuit 10 is too low or too high, rendering circuit 10 unable to control a brown-out block state, i.e. the power supply voltage is unsuitable for proper operation.

Output drive stage 41 is included to provide a delay so that latch 22 and delay elements 24 and 26 are reset, and current source 52 is enabled before a fault signal is provided to the rest of the power supply control circuitry. It should be noted that current source 52 is in an enabled configuration when a fault is detected.

FIG. 3 is a schematic diagram of a circuit 100 suitable for use in power supply applications in accordance with additional embodiments of the present invention. Circuit 100 is similar to circuit 10 except that in circuit 100 operational amplifier 20 is replaced by a circuit element such as, for example, a comparator 102 and clamping circuit 32 is replaced by clamping circuit 104. Because of these differences, the brown-out detection circuit of FIG. 3 has been identified by reference character 12A, i.e., the letter A has been appended to reference character 12 to distinguish the brown-out detection circuits of FIGS. 1 and 3 from each other. More particularly, comparator 102 has a non-inverting input coupled for receiving a reference potential V_(REF), an inverting input connected to brown-out node 16, and an output commonly connected to an input of latch 22 and to an input of logic circuit 28. The connection of the output of comparator 102, the input of latch 22, and the input of logic circuit 28 form a node 23. Clamping circuit 104 is comprised of NPN bipolar transistors 106 and 108 having base terminals commonly connected together and collector terminals commonly connected together. The emitter terminal of NPN bipolar transistor 106 is connected to the inverting input terminal of comparator 102 and the emitter terminal of NPN bipolar transistor 108 is coupled for receiving a clamping voltage V_(CLAMP) from a voltage source 110. A resistor 112 is coupled between the commonly connected base terminals and the commonly connected collector terminals. The commonly connected collector terminals are coupled for receiving a source of operating potential such as, for example, a voltage V_(DD). Similar to circuit 10, auxiliary clamping circuit 34 is included to clamp brown-out node 16 at a voltage level that is below reference voltage V_(REF) when the voltage level of the supply voltage for circuit 10 is too low or too high, rendering circuit 10 unable to control a brown-out block state, i.e. the power supply voltage is unsuitable for proper operation.

FIG. 4 is a flow diagram 150 illustrating the operation of brown-out detection circuit 12A in accordance with embodiments of the present invention. A voltage V_(BO) that is a portion of an input voltage from an input stage 11 is applied to brown-out node 16 (indicated by box 152). Voltage V_(BO) is compared with a reference voltage V_(REF) by comparator 102, which generates a comparison voltage V_(COMP) at its output, i.e., node 23, (indicated by box 154). By way of example, reference voltage V_(REF) is one volt. If voltage V_(BO) is less than reference voltage V_(REF), comparator output signal V_(COMP) is a logic high voltage and if voltage V_(BO) is greater than reference voltage V_(REF), comparator output signal V_(COMP) is a logic low voltage.

In response to comparison voltage V_(COMP) being a logic low voltage, brown-out detection circuit 12A continues to compare brown-out voltage V_(BO) to reference voltage V_(REF) and generates or updates comparison voltage V_(COMP) (indicated by box 154 and decision diamond 155). If comparison voltage V_(COMP) is a logic high voltage, latch 22, i.e., voltage V_(LATCH) at node 25, is set to a logic high voltage (indicated by decision diamond 155 and box 156). In addition, each time the output (node 25) of latch 22 transitions to a logic high voltage level, a blanking period is started (indicated by box 157). As discussed above, the length of the blanking delay may be, for example, 100 msec. However, the duration or length of the blanking delay is not a limitation of the present invention. The voltage at node 29 is a logic low voltage during the blanking time and turns to a logic high voltage at the end of the blanking time. The blanking time is a predetermined delay that precludes any transient changes in voltage V_(BO) from being detected and identified as a fault or brown-out condition. At the end of the blanking time, brown-out detection circuit 12A starts a monitoring window during which comparison voltage V_(COMP) is monitored (indicated by box 158). If comparison voltage V_(COMP) is low for the whole monitoring window (indicated by decision diamond 159), brown-out detection circuit 12A resets latch 22 and delay elements 24 and 26 (indicated by box 160) after the monitoring window ends. Then brown-out detection circuit 12A continues to compare brown-out voltage V_(BO) with reference voltage V_(REF) and generates or updates comparison voltage V_(COMP) (indicated by box 154 and decision diamond 155). If comparison voltage V_(COMP) is high at any time during the monitoring window, brown-out detection circuit 12A closes switch 50, resets latch 22, and resets delay elements 24 and 26 (indicated by box 161). Thus, latch 22 is reset at the end of the monitoring window delay or if a brown-out fault or condition has occurred. The voltage level at node 29 indicates whether a monitoring window is operating within brown-out detection circuit 12A. The voltage at node 29 is at a logic high voltage level during the monitoring window and a logic low voltage level during the rest of the time, including during the blanking delay. The voltage at node 29 and the comparison voltage V_(COMP) are monitored and a brown-out condition occurs if the voltage being monitored at node 29 and the comparison voltage results from voltage V_(BO) being less than the reference voltage V_(REF).

If reference voltage V_(BO) is less than voltage V_(REF), NPN bipolar transistor 106 turns on, resulting in voltage source 110 clamping the inverting input of comparator 102 at voltage level V_(CLAMP). It should be noted that clamping voltage V_(CLAMP) is a minimum voltage that is forced on brown-out node 16. By way of example, reference voltage V_(REF) is about 1 volt and clamping voltage V_(CLAMP) is about 980 millivolts (mV). The voltage level of clamping voltage V_(CLAMP) is selected so that it is lower than reference voltage V_(REF). Thus, comparator output signal V_(CLAMP) is at a logic high voltage level during clamping of the brown-out voltage V_(BO). Closing switch 50 enables current source 52 to conduct current I_(HYS), which provides programmable hysteresis. By way of example, current source I_(HYS) conducts a current of about 7 microamperes.

Output drive stage 41 is included to provide a delay so that the output of comparator 102 can be separated from brown-out pin 16 when latch 22 and delay elements 24 and 26 are reset and current source 52 is enabled.

By now it should be appreciated that brown-out detection circuits and a method for detecting the occurrence of a brown-out fault or a brown-out condition have been provided. If a voltage at the input of the brown-out detection circuit falls below a reference voltage, a comparison signal V_(COMP) having a logic high voltage level is generated. A delay or blanking time is introduced into the brown-out detection signal path to allow for short interruptions of the mains. Another delay time is included after the blanking delay to serve as a monitoring window. If comparison signal V_(COMP) becomes a logic high voltage during the monitoring window, a fault is detected.

Although specific embodiments have been disclosed herein, it is not intended that the invention be limited to the disclosed embodiments. Those skilled in the art will recognize that modifications and variations can be made without departing from the spirit of the invention. It is intended that the invention encompass all such modifications and variations as fall within the scope of the appended claims. 

What is claimed is:
 1. A brown-out detection circuit, comprising: a circuit element having first and second inputs and an output, the circuit element capable of comparing at least two signals; a latch having first and second inputs and an output, the first input of the latch coupled to the output of the circuit element; a first delay element having an input coupled to the output of the latch; a second delay element having an input coupled to the output of the first delay element and an output coupled to the second input of the latch; and a logic circuit having at least first and second inputs and an output, the first input coupled to the output of the first delay element and the second input coupled to the output of the circuit element.
 2. The brown-out detection circuit of claim 1, further including a clamping circuit coupled to the second input of the circuit element.
 3. The brown-out detection circuit of claim 1, wherein the clamping circuit comprises: first and second transistors, each of the first and second transistors having a control electrode and first and second current carrying electrodes, the control electrodes of the first and second transistors coupled together and the first current carrying electrodes of the first and second transistors coupled together; a resistor having first and second terminals, the first terminal coupled to the control electrodes of the first and second transistors and the second terminal coupled to the first current carrying electrodes of the first and second transistors; and a voltage source coupled to the second current carrying electrode of the second transistor.
 4. The brown-out detection circuit of claim 1, further including a hysteresis circuit coupled to the second input of the circuit element.
 5. The brown-out detection circuit of claim 4, wherein the hysteresis circuit comprises a current source having first and second terminals, the first terminal coupled to the second input of the circuit element and a switch having a first terminal coupled to the second terminal of the current source, and a second terminal coupled for receiving a source of operating potential.
 6. The brown-out detection circuit of claim 1, wherein the logic circuit comprises a first gate having first and second inputs and an output, wherein the first input serves as the first input of the logic circuit and a second gate having first and second inputs, the first input of the second gate coupled to the output of the circuit element and the second input of the second gate coupled to the output of the first gate.
 7. The brown-out detection circuit of claim 1, wherein the circuit element comprises one of an operational amplifier or a comparator. 